Various auxiliary machines mounted in automobile engines are driven by an engine crankshaft through a belt. Amongst the auxiliary machines, an alternator is driven by an engine crankshaft so as to generate electricity. When the alternator is coupled with the engine crankshaft so as to simultaneously rotate, as the number of rotation of the crankshaft decreases, the power generating efficiency decreases. In order to prevent the decrease of the power generating efficiency, an alternator having a one-way clutch in a pulley portion thereof has been devised. As to the alternator, when the rotational speed of the crankshaft lowers, the rotation of a rotor of the alternator is continued by inertial force so as to improve the power generating efficiency thereof.
FIG. 10 shows a pulley unit. The pulley unit is equipped with a pulley 1, a rotor shaft 2, a one-way clutch 3 and a deep groove ball bearing 44 and a needle roller bearing 45. A cam face 10 is formed intermediate the axial direction of an outer peripheral surface of a rotor shaft 2. A belt 6 is wound around an outer periphery of the pulley 1. When the belt 6 is driven by an engine crankshaft of an automobile not shown in a figure, the pulley 1 is rotationally driven. The rotor shaft 2 is fixed to a rotor of an alternator. The one-way clutch 3 has a cage 12 and a plurality of rollers 13 which are housed in a plurality of pockets of the cage 12 one each. The rollers 13 are always pushed to a narrow side (lock side) of a wedge-like space by a coil spring not shown in a figure. The wedge-like space is an annular space in a circumferential direction formed between an inner peripheral surface of an outer ring 11 and the cam face 10 of the rotor shaft 2 so as to roll the rollers 13. The wedge-like space is narrowed to the lock side. The one-way clutch 3, the deep groove ball bearing 44 and the needle roller bearing 45 are sealed by a pair of seal rings 20, 21 and a seal annular body 22 and lubricated by a common lubricant.
In operation, when a rotational speed of the pulley 1 becomes relatively faster than the same of the rotor shaft 2, the rollers 13 of the one-way clutch 3 are rolled to the narrow side of the wedge-like space and so as to be in a locked state. The locked state is a state in which the outer ring 11 and the rotor shaft 2 can be integrally rotated via the rollers 13. The outer ring 11 and the pulley 1 are integrally arranged. As a result, the pulley 1 and the rotor shaft 2 are integrally rotated in the locked state so that a rotational power can be transmitted from the pulley 1 to the rotor shaft 2.
When the rotational speed of the pulley 1 becomes relatively slower than the same of the rotor 2, the rollers 13 of the one-way clutch 3 are rolled to a broad side (free side) which is the opposite side from the narrow side of the wedge-like space. As a result, the pulley unit becomes in a free state. The free state is a state in which the outer ring 11 and the rotor shaft 2 can freely rotate to each other. Thus, transmission of a rotational power from the pulley 1 to the rotor shaft 2 is intercepted. In the case where the rotor shaft 2 is being rotated prior to the interception, the rotor shaft 2 continues its rotation only by its own rotational inertial force after the interception.
Referring to FIGS. 10 and 11, the deep groove ball bearing 44 will be described. The bearing 44 takes an axial load. The bearing 44 is equipped with an inner ring 50, an outer ring 51, a cage 56 and a plurality of balls 52. The inner ring 50 and the outer ring 51 respectively have raceway grooves 53 and 54. The race grooves 53 and 54 correspond to an arc of a circle 55 indicated by an imaginary line larger than a diameter of the balls 52. In a case that a curvature radius of the raceway grooves 53 and 54 is R and a diameter of the balls 52 is D, a raceway curvature (%) of the raceway grooves 53 and 54 is expressed as the formula (1).Raceway curvature=(R/D)×100   (1)
According to the formula (1), when the raceway curvature is 50%, the radius of the balls 52 (D/2) and the curvature radius (R) of the raceway grooves 53 and 54 are identical. Therefore, when the raceway curvature is 50%, the balls 52 are fitted to the raceway groves 53 and 54 with no gap. In this manner, when the raceway curvature is 50%, an axial deviation of the bearing 11 becomes completely nil. In this state, the balls 52 are fitted to the raceway groves 53 and 54 with no gap. Therefore, there are possibilities of a generation of a seizure on the raceway grooves 53 and 54 and of an undesirable influence on a service life of a lubricant due to a contact friction between the balls 52 and the raceway grooves 53 and 54. In order to deal with this, the above mentioned raceway curvature is set to a range of 52.0–52.5% for the raceway groove 53 of the inner ring 50, and to a range of 53.0–53.5% for the raceway groove 54 of the outer ring 51.
As shown in FIG. 12, the bearing 44 needs to be mounted by force-fitting between the pulley 1 and the rotor shaft 2. For the reason thereof, a radial internal clearance of the bearing 44 is set to a few times as large as a standard radial internal clearance with regard to a tolerance. For example, in case of the bearing 44 having bearing designation 6807 in JIS (Japan Industrial Standard), a radial internal clearance before mounting thereof is set to 50–100 μm. When the bearing 44 is force-fitted between the pulley 1 and the rotor shaft 2, the radial internal clearance decreases. Therefore, the radial internal clearance after mounting of the ball bearing 44 is set to a range of approximately 21–52 μm.
In the case where the deep groove ball bearing 44,in which the raceway curvature of the raceway groove 53 is a range of 52.0–52.5%, the raceway curvature of the raceway groove 54 is a range of 53.0–53.5%, and the radial internal clearance before mounting thereof is a range of 75–99 μm, is mounted by force-fitting between the pulley 1 and the rotor shaft 2, the radial internal clearance after mounting thereof is a range of 21–52 μm and the axial internal clearance thereof is a range of 140–239 μm, as shown in Table 7.
TABLE 7radial internalclearance afteraxial internalmounting [μm]clearance [μm]21~52140~239
The axial internal clearance, as shown in FIG. 13, denotes the axial deviation of the inner and outer rings 50 and 51 based on raceway curvatures and the radial internal clearance after mounting of rack grooves 53 and 54.